1. Field of the Invention
This invention relates to a power steering device for a four-wheel drive vehicle, which generates a steering reaction force appropriate to the variably changing front and rear driving force distribution.
2. Description of the Related Art
Various types of power steering device for a vehicle have been developed for the purpose of providing a driver with an appropriate steering reaction force.
For example, Japanese Utility Model Application Publication No.H2-43765 discloses a power steering control device for a vehicle, in which controlling is achieved by estimating the angle of a sideslip generated as the vehicle is steered based on a movement model of the vehicle using the vehicle velocity and steering angle; and determining a target value of the steering reaction force on steering wheels based on the vehicle velocity and sideslip angle so that the steering reaction force with this target value is generated.
It is generally known that a self-aligning torque changes when braking/driving forces change; the change in self-aligning torque in turn causes the steering reaction force to change. Therefore, when the front and rear driving force distribution is variably controlled in a four-wheel drive vehicle, the self-aligning torque changes accordingly, resulting in the change in steering power and giving the driver an unpleasant sensation during steering. For example, when the front wheel driving force increases, the self-aligning torque on the front wheel side decreases; thus, the steering power becomes less, giving the driver an unpleasant sensation. Since such effects of the self-aligning torque are not considered in the technology of the above-mentioned patent literature, adapting the technology for a four-wheel drive vehicle poses a problem of giving the driver an unpleasant sensation during steering.